1. Field of the Invention
The present invention relates to a novel process for producing alkene oxide. More particularly, the present invention relates to the addition of carboxylic-group and/or carboxylate-group containing materials in the electrolysis of bromide, water, and alkenes during the production of alkene oxides.
2. Description of the Prior Art
Propylene oxide is a bulk commodity chemical used mainly to make polyols and polyethers for urethane polymers and to make propylene glycol for unsaturated polyester resins. But present manufacturing processes use raw materials inefficiently, produce large amounts of coproducts, produce a large waste and pollution burden, and/or use large amounts of energy.
The hydroperoxide coproduct processes are a common method of manufacturing propylene oxide and are exemplified by the tertiary butanol process. In this process, isobutane is oxidized by molecular oxygen to a tertiary butyl hydroperoxide, which thereafter reacts with propylene to form propylene oxide and tertiary butanol. One drawback to this process is that it produces an equal or greater amount of coproducts.
Another process is where hydrogen peroxide is reacted with recycled propionic acid to form perpropionic acid, which thereafter reacts with propylene to form propylene oxide. However, the high cost of hydrogen peroxide makes this process uneconomical in most circumstances.
Peracetic acid, produced by reacting acetaldehyde with ethyl acetate and air, can be used to epoxidize propylene but gives over one ton of coproduct acetic acid per ton of propylene oxide.
The direct oxidation of propylene by molecular oxygen to propylene oxide gives only 5-70% propylene efficiency and many undesirable byproducts. It should be noted that this route has never been commercialized.
The conventional process is the chlorohydrin process, wherein propylene reacts with chlorine in acidic water to form propylene chlorohydrin and 1,2-dichloropropane. Then, in a separate dehydrochlorination step, the chlorohydrin is contacted with a lime slurry to produce propylene oxide and calcium chloride. The major drawback to this process is that, for each ton of propylene oxide, 0.1 to 0.2 tons of propylene dichloride are produced, and typically 2 tons of calcium chloride in 43 tons of waste water must be disposed of.
To alleviate the chloride disposal problem, the conventional chlorohydrin process can be integrated with a dedicated conventional chlorine/caustic plant, but the NaCl brine leaving the chlorohydrin plan is too dilute to feed directly to chlorine cells. Since concentrating the brine is too expensive, after adding NaCl only part of it can be recycled to the dedicated chlorine plant; the remainder must be consumed in another large chlorine plant. Therefore propylene oxide and organic byproducts must be completely steam stripped from the brine, and this consumes an extraordinary amount of energy.
To eliminate brine loss and dependence on a second chlorine plant, total-NaCl brine-recycle processes have been proposed. These use conventional chlorine diaphragm or membrane cells but make propylene chlorohydrin by reacting propylene directly with the chlorine dissolved in the acidic anolyte. The anolyte is then mixed with a highly basic catholyte to swing the pH to convert the propylene chlorohydrin to propylene oxide and chloride. The efficiency is similar to the conventional process. After the products are removed, the total brine is recycled to the chlorine cell, after replacing chloride lost due to 1,2-dichloropropane and bis(chloropropyl) ether formations.
The in situ generation of propylene oxide using a chlorine cell has several disadvantages as well as the stated advantages. The cells must be operated at elevated temperature, typically 50.degree.-60.degree. C. This severely reduces propylene solubility and thus reduces the obtainable propylene oxide concentration, which also makes product recovery more difficult. Elevated temperature also increases propylene glycol formation by propylene oxide hydrolysis. Typically the cells must be operated at less than half the electrical current density of chlorine plants and this significantly increases the capital cost per unit of chlorine generated. Furthermore, the in situ generation of propylene oxide does not work at all in an undivided cell, because rapid formations of propylene chlorohydrin and propylene oxide are not compatible at any common pH. Chlorate formation is also prohibitive in an undivided cell. Therefore a membrane or diaphragm is needed, but since these materials are costly, divided cells can be expensive to build, the capital costs being very high. A membrane or diaphragm can significantly increase the voltage drop across a cell and hence increase the electrical power consumption. Also, the very large pH gradient across the diaphragm or membrane promotes fouling when organics are in the brine. Formation of a separate 1,2-dichloropropane phase must be avoided because both chlorine and propylene dissolve into it and form more 1,2-dichloropropane selectively.
Bromide electrolysis avoids the disadvantages of a chlorine-based system. Propylene oxide can be made in a simple, closed process that uses electrolysis to make bromine from bromide at the anode and hydroxide (and hydrogen) from water at the cathode. Propylene reacts with bromine and then water to form propylene bromohydrin, which is dehydrobrominated by hydroxide, regenerating the bromide, to give propylene oxide. The main byproduct is 1,2-dibromopropane and a minor amount of propylene glycol forms. Between pH 8 and 11, propylene bromohydrin and propylene oxide form rapidly at the same pH. Therefore a diaphragm or membrane is not needed and all reactions can occur within the undivided reactor. This simplifies reactor design and considerably lowers reactor capital costs. The lower electrolysis voltage of bromine and the absence of a diaphragm or membrane reduce the voltage drop across the cell and hence reduce electrical power consumption. Because bromine is more reactive than chlorine, reaction with propylene is rapid at and below room temperature. Operating at lower temperatures increases the propylene solubility which in turn increases propylene oxide concentration, all of which helps product recovery. Formation of 1,2-dibromopropane within any 1,2-dibromopropane phase present is insignificant because the free-bromine concentration is very low. Therefore, the reactor can be operated with an electrolyte saturated in 1,2-dibromopropane, which can then be removed easily by settling. Propylene efficiency is similar to conventional chlorine-based systems.
It has now been discovered that by adding carbonate and/or bicarbonate to the bromide electrolyte, the formation of 1,2-dibromopropane is effectively reduced in making propylene oxide in situ using bromide electrolysis in an undivided reactor. This occurs independently of any pH effect (carbonate and bicarbonate form a buffer, which is desirable to control pH in industrial scale reactors).